GBT X-band (9 GHz): Pointing Stability

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Scarlett Simpson
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1 GBT X-band (9 GHz): Pinting Stability Dana S. Balser, Rnald J. Maddalena, Frank Ghig, & Glen I. Langstn 10 March 2002 Abstract The pinting stability f the GBT is evaluated by tracking the half-pwer pint f 3C123 ver a duratin f 30min ffset in bth azimuth and elevatin during benign weather cnditins. The rms variatins f the telescpe beam n the sky are n the rder f 1", althugh peak-t-peak variatins as large as 5" are present. A pwer spectrum analysis reveals natural frequency mdes less than 0.5 Hz which are significantly lwer than predicted by thery r measured by laser ranging and accelermeters. 1 Intrductin The pinting stability f the GBT is tested using the X-band receiver. The pinting stability is prbed by tracking the half-pwer psitin f a pint surce. The brightness distributin f a pint surce is apprximately a Gaussian. The maximum slpe f a Gaussian ccurs at the half-pwer psitin where variatins in the system temperature are mst sensitive. This prcedure has been discussed in GBT Cm. Mem 9 fr bservatins at S-band (2 GHz). The bservatins and results are discussed in 2. The cnclusins are in 3. 2 bservatins and Results 2.1 Half-Pwer Pint Test The half-pwer pint test prvides a sensitive prbe f hw well the telescpe's beam tracks a specified psitin with time. It therefre nt nly tests the pinting mdel but is als sensitive t prblems in the fcus tracking. Fr example, significant mvements in the feedarm shuld be detected. These bservatins were perfrmed with the new fcus trakcing

GBT Cm. Mem 20 GBT X-band (9 GHz): Pinting Stability Page 2 Table 1: Lcked Rtr Frequencies^ Mde Frequency (Hz) Descriptin 1 0.57 TVanslatin alng elevatin axis 2 0.

2 GBT Cm. Mem 20 GBT X-band (9 GHz): Pinting Stability Page 2 Table 1: Lcked Rtr Frequencies^ Mde Frequency (Hz) Descriptin TVanslatin alng elevatin axis Cupled psitive azimuth and psitive elevatin rtatins Cupled psitive azimuth and negative elevatin rtatins Secnd elevatin rtatin mde with psitvie feed-arm and negative reflectr x-axis rtatin Azimuth and crss-elevatin rtatin mde with psitve elevatin rating structure and negative alidade y-axis rtatins Azimuth rtatin with sme ppsite y-axis rtatin f feed-arm and reflectr structures Third elevatin mde with feed-arm bending and alidade vertical translatin t Taken frm Lral Tech. Mem 49. mdel which included tilts f the sub-reflectr (GBT Cm. Mem 18), a refined mdel fr refractin (GBT Cm. Mem 16), and a new pinting mdel (GBT Cm. Mem 17). The pinting stability was measured n 2002 January 20 (prject pnt_highgreg_10) by tracking the half-pwer psitin f 3C123 by ffsetting in azimuth and elevatin by half f the FWHM beamwidth (0'.7). Befre each track the G prcedure peak was used t update the lcal pinting crrectins (LPCs). Then the G prcedure track was used t track the half-pwer psitin fr 30 min ffset 'J in elevatin and then fr 30 min ffset 0!7 in azimuth. The results are summarized in Figures 1 and 2 where the antenna temperature and the wind speed are shwn fr the half-pwer pint tracks ffset in azimuth and elevatin, respectively. nly the RCP channel is shwn. Ntice that the wind speed is typically less than 5 miles per hur during the entire perid. Using Equatin (2) f GBT Cm. Mem 9, the antenna temperature is cnverted int arcsec n the sky using T 0 = 4.2 (in units f TCAL), and A = 1'.4. Figures 3 and 4 shw the ffset beam psitin as a functin f time. The antenna elevatin was ~ An rms f 1''3 and 1"0 are determined fr the ffset psitin in azimuth and elevatin, respectively. The peak-t-peak variatins are as large as 5 arcsec. The rms variatins in the ffset psitin are abut a factr f tw smaller than previus measurements at S-band (GBT Cm. Mem 9). This is prbably due t (i) imprvements in the fcus tracking and pinting mdels; and (ii) the benign weather cnditins. Because the sample perid was set t 0.15 secnds the fundamental mdes f the feed-arm

GBT Cm. Mem 20 GBT X-band (9 GHz): Pinting Stability Page 3 C -. C 3 2 1 b^^^^ i -i 1 1 1 r S l 3C123 (Scan 51) 1500 5000 Sample l' Figure 1: Half-pwer pint track f 3C123 in azimuth.

3 GBT Cm. Mem 20 GBT X-band (9 GHz): Pinting Stability Page 3 C -. C b^^^^ i -i r S l 3C123 (Scan 51) Sample l' Figure 1: Half-pwer pint track f 3C123 in azimuth. The antenna psitins was ffset 0'.7 in azimuth. The tp panel plts the wind speed while the bttm panel plts the antenna temperature in units f TCAL- The sample perid was 1 and 0.15 secnds fr the weather statin and digital cntinuum receiver, respectively, fr a ttal time f 30 min. can be analyzed. The predicted GBT mdes are shwn in Table 1 which is reprduced frm Lral Tech. Mem 49. The results in Table 1 are based n a finite element analysis using the NISA II structural analysis cde. The lwest natural frequency is 0.57 Hz. Figures 5 and 6 shw the pwer spectra fr the azimuth and elevatin series, respectively. The lwer panels f each figure shw an expanded scale. The mdes measured in azimuth are 0.12, 0.63, and 0.85 Hz. The mdes measured in elevatin are 0.15, 0.30, and 1.14 Hz. Ntice that mst f the pwer is cntained in mdes which are at frequencies significantly lwer than 0.5 Hz. The mdes abve 0.5 Hz prbably crrespnd t the mdes in Table 1. Vibratins in the feed-arm were analyzed using laser ranging and accelermeters (GBT Mem 204). The lwest bserved frequency was 0.7 Hz althugh the telescpe was nt yet cmplete and the ttal mass was smaller at that time. Als the telescpe was nt tracking but lcked dwn. D the mdes belw 0.5 Hz crrespnd t prblems in the serv system? 3 Cnclusin Under benign weather cnditins the rms variatins f the telescpe's beam is n the rder f 1" with variatins as large as 5 //. A pwer spectrum analysis reveals significant pwer in vibratinal mdes with frequencies less than 0.5 Hz.

4 GBT Cm. Mem 20 GBT X-band (9 GHz): Pinting Stability Page C123 (Scan 52) 5000 Sample l Figure 2: Half-pwer pint track f 3C123 in elevatin. The antenna psitins was ffset 0'.7 in azimuth. The tp panel plts the wind speed while the bttm panel plts the antenna temperature in units f TCAL- The sample perid was 1 and 0.15 secnds fr the weather statin and digital cntinuum receiver, respectively, fr a ttal time f 30 min. A - in 2 -, c Q 'tn a> (n -2-4 J L J t_ Time (secnds) 1500 Figure 3: The ffset psitin in azimuth f the beam in arcsec as a funtin f time.

5 GBT Cm. Mem 20 GBT X-band (9 GHz): Pinting Stability Page 5 a> 2 r, c 'vi Q_ - -' a> Time (secnds) 1500 Figure 4: The ffset psitin in elevatin f the beam in arcsec as a funtin f time.

6 GBT Cm. Mem 20 GBT X-band (9 GHz): Pinting Stability Page 6 10' Pwer Spectrum (Azimuth ffset) CL Frequency (Hz) Pwer Spectrum (Azimuth ffset) Frequency (Hz) 1.5 Figure 5: Pwer spectrum f the half-pwer pint time series in azimuth. The upper and lwer panels are the same data with different scales.

7 GBT Cm. Mem 20 GBT X-band (9 GHz): Pinting Stability Page 7 10' Pwer Spectrum (Elevatin ffset) T 1! r Q_ Frequency (Hz) i,. I,1 L Pwer Spectrum (Elevatin ffset) a Frequency (Hz) 1.5 Figure 6: Pwer spectrum f the half-pwer pint time series in elevatin. The upper and lwer panels are the same data with different scales.

Laser Rangefinder Deflection Measurements

Laser Rangefinder Deflection Measurements Laser Rangefinder Deflectin Measurements f the GET Derrick Amy L. Petticrew Nvember 18, 1996 1 Intrductin Sectin B f the left hrizntal feedarm was suspended fr weighing and lifted int place n Octber 31